This invention relates generally to a protection circuit for electrical apparatus and particularly to a monitoring system for determining if the ground terminal of a three-wire grounded AC power source for the electrical apparatus is in fact properly grounded. While the invention may be employed with any electrical apparatus or equipment having a grounded chassis, it is especially attractive when used with an adjustable motorized hospital bed and will be described in that environment.
A motor operated hospital bed, the positioning of which may be remotely controlled by means of an electrical control system operated by the patient, must be completely safe to preclude any possibility of the patient being subject to hazardous electrical shocks from the voltages present in the control system or in the conductors supplying AC power thereto. To that end, and in accordance with conventional practice, the metal bed frame or chassis is usually connected to the ground terminal of the three-wire AC power source in order that the bed chassis will be tied to earth ground. In this way, circuit component failures or insulation breakdowns cannot establish the bed frame at a dangerously high potential relative to earth ground. Of course, maintaining a patient immune to or protected against malfunctioning or faults, that tend to cause the application of undesired voltages to the bed chassis, requires that the bed frame remain properly grounded at all times and this means that the ground terminal of the grounded AC power source must remain connected to earth ground via a low-resistance connection.
Monitoring circuits have been developed for testing or checking the integrity of the ground connection at the ground terminal, but these prior arrangements have many shortcomings and disadvantages, particularly when employed with hospital beds. For example, in some of the ground integrity monitors developed heretofore, RF (radio frequency) signals are used to sense the quality of the ground connection, but this method is not always accurate since the presence of any inductance may falsely indicate a bad ground connection when in fact it may be a good connection. Moreover, the hospital bed may contain leakage monitoring equipment to detect undesired leakage currents, and that equipment may be falsely operated by the RF signals. In addition, the RF energy is likely to deleteriously affect some of the other electrical equipment in the hospital where the bed is located. Other ground testers effectively function like ohmmeters but they usually cannot detect very low resistances, such as less than three ohms, which is the value desired in a good ground connection. Besides, any noise signals present will affect the operation of an ohmmeter-type ground integrity monitor and lead to erroneous results.
The ground integrity monitor of the present invention constitutes a significant improvement over those developed heretofore, since it is, inter alia, of relatively inexpensive construction, requires very little power, provides accurate results, is capable of detecting very low resistance ground connections, does not employ RF signals, is noise immune, and will not affect the operation of other equipment.